Pressure-sensitive adhesives (PSAs) are known to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Materials that have been found to function well as PSAs include polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. PSAs are characterized by being normally tacky at room temperature (e.g., 20° C.). PSAs do not embrace compositions merely because they are sticky or adhere to a surface.
Polyurethane PSAs have previously been made, for instance by adding tackifier resins and/or plasticizers to a polyurethane base polymer. Alternatively, polyurethane PSAs have been made by undercrosslinking of the polyurethane. Both of these methods result in PSAs having migratable components in the PSA. Polyurethanes have also been made via methods involving moisture curing or complex formulations.
There remains a need for PSAs based on chemically crosslinked polyurethanes, having advantageous adhesive properties, low glass transition temperatures, and cohesion following thermal cycling.